A plant ecological evaluation of mechanical bush thinning in Marakele Park, Limpopo Province

[摘要] English: Bush encroachment is currently of great concern in Marakele Park. The main motivation for thisstudy, conducted during the 2003/2004 and 2004/2005 seasons, was to determine whethermechanical bush thinning, executed with a mechanical mulc her, namely the Barko Tractor, wassuccessful in solving the bush encroachment problem in the areas where it was applied. Thespecific objectives of this study were to identify, describe and interpret the plant communities of asection of Marakele Park, and to establish the influences of the thinning treatments on thedynamics of the ecosystem, more specifically the regrowth and browse production of the woodyplants, the species composition and dry matter (DM) yield of the herbaceous layer and the shortterm changes in the soil.Eight experimental plots (3 treatments, 3 controls, 2 coppice) were selected in three veld types(Acacia mellifera �?Grewia flava, Combretum apiculatum �?Grewia flava and Acacia erubescens�?Dichrostachys cinerea), in which tree thinning was applied during 2002 and 2003. Each plotwas 100 m x 200 m (20 000 m2 = 2 ha) in size. The vegetation of the plots wasphytosociologically studied during the 2003/2004 season with the aid of the Braun-Blanquetvegetation sampling method. A total of 80 relevés were surveyed and upon analysis 3 majorcommunities, 7 communities, 6 sub-communities and 3 variants were identified. The woodylayer was quantified with a quantitative description technique, which is incorporated in theBECVOL-model. A step point-method and the Ecological Index Method were used to determinethe species composition and veld condition of the herbaceous layer, respectively, and a harvestingmethod was used to determine the DM yield and the associated grazing capacity. The thinning treatments resulted in an initial decreased number of woody plants, but since nofollow-up treatments were applied, a large number of new seedlings have since established andthe majority of cut-plants coppiced vigorously. However, approximately three years after thethinning treatments the leaf biomass (ETTE ha-1) of the woody layer increased due to regrowthand re-encroachment to a point where negative interactions between the woody and herbaceousplants, as a result of competition for soil water and nutrients, were evident again.The species diversity of the herbaceous layer increased after the thinning treatments, but speciesnormally associated with disturbed and overgrazed veld still dominated. It was concluded thatthe herbaceous layer of Marakele Park is in a poor ecological state and indications are that it isstill deteriorating. The herbaceous DM yield did not respond to the thinning treatments asexpected, except in areas protected from grazing herbivores. As a result, the grazing capacity ofthe Park was comparatively low. It is clear that Marakele Park is currently overstocked withvarious game species, especially of high density, selective short grass grazers. Thinningtreatments will therefore not be successful unless the herbivore game numbers are reduced.Few soil variables changed significantly as a result of the thinning treatments, but in view of thefact that soil enrichment is a slow process, monitoring of the soil variables is recommended. Thespecific soil propertie s did, however, have a decisive influence on the vegetation type. TheCombretum apiculatum �?Grewia flava plots occurred on relatively shallow, gravelly soil, whilethe Acacia dominated plots occurred on deeper, more fertile soil. It was also concluded that theBarko Tractor, due to its size and weight, had a negative impact on the soil, mainly in the form ofsoil compaction.Subhabitat differentiation, rainfall and herbivory played an important role in the study area andindicated that the vegetation of Marakele Park displays both equilibrial and non-equilibrial trends.The high grazing pressure, together with the high incidence of coppice and re-encroachment ofwoody plants after the initial thinning treatments, as well as the lack of follow-up treatments,effectively neutralised the success of the mechanical bush thinning treatments. In order to restorethese thinned areas from re-encroachment it will be necessary to cut the plants again and combinethis effort with a cut-stump treatment (chemical herbicide). Due to the negative impact of the Barko Tractor, it should not be used again during any follow-up operation.

[发布机构] University of the Free State